The ventilation circuits of aircraft incorporate fans for circulating air in the ventilation ducts. These fans rotate at high speed and must be highly reliable. To meet these requirements, the shaft supporting the fan impeller is supported on two ball bearings. These bearings are lubricated to prevent their heating and rapid destruction.
The degradation of a ball bearing causes the fan to produce fumes and odours which are dispersed into the aircraft cabin. It is therefore useful to be able to detect such a degradation of a fan bearing rapidly, and preferably before the fan produces fumes and odours.